How old are the oldest stars? Using ESO’s VLT, astronomers recently  measured the age of a star located in our Galaxy. The star, a real  fossil, is found to be 13.2 billion years old, not very far from the  13.7 billion years age of the Universe. The star, HE 1523-0901, was  clearly born at the dawn of time. 

“Surprisingly, it is very hard to pin down the age of a star”, the  lead author of the paper reporting the results, Anna Frebel,  explains. “This requires measuring very precisely the abundance of  the radioactive elements thorium or uranium, a feat only the largest  telescopes such as ESO’s VLT can achieve.”

This technique is analogous to the carbon-14 dating method that has  been so successful in archaeology over time spans of up to a few tens  of thousands of years. In astronomy, however, this technique must  obviously be applied to vastly longer timescales.

For the method to work well, the right choice of radioactive isotope  is critical. Unlike other, stable elements that formed at the same  time, the abundance of a radioactive (unstable) isotope decreases all  the time. The faster the decay, the less there will be left of the  radioactive isotope after a certain time, so the greater will be the  abundance difference when compared to a stable isotope, and the more  accurate is the resulting age.

Yet, for the clock to remain useful, the radioactive element must not  decay too fast – there must still be enough left of it to allow an  accurate measurement, even after several billion years.

“Actual age measurements are restricted to the very rare objects that  display huge amounts of the radioactive elements thorium or uranium,”  says Norbert Christlieb, co-author of the report.

Large amounts of these elements have been found in the star HE  1523-0901, an old, relatively bright star that was discovered within  the Hamburg/ESO survey [1]. The star was then observed with UVES on  the Very Large Telescope (VLT) for a total of 7.5 hours.

A high quality spectrum was obtained that could never have been  achieved without the combination of the large collecting power  Kueyen, one of the individual 8.2-m Unit Telescopes of the VLT, and  the extremely good sensitivity of UVES in the ultraviolet spectral  region, where the lines from the elements are observed.

For the first time, the age dating involved both radioactive elements  in combination with the three other neutron-capture elements  europium, osmium, and iridium.

“Until now, it has not been possible to measure more than a single  cosmic clock for a star. Now, however, we have managed to make six  measurements in this one star”,” says Frebel.

Ever since the star was born, these “clocks” have ticked away over  the eons, unaffected by the turbulent history of the Milky Way. They  now read 13.2 billion years.

The Universe being 13.7 billion years old, this star clearly formed  very early in the life of our own Galaxy, which must also formed very  soon after the Big Bang.

  More Information

This research is reported in a paper published in the 10 May issue of  the Astrophysical Journal (“Discovery of HE 1523-0901, a Strongly r- Process Enhanced Metal-Poor Star with Detected Uranium”, by A. Frebel  et al.).

The team includes Anna Frebel (McDonald Observatory, Texas) and John  E. Norris (The Australian National University), Norbert Christlieb  (Uppsala University, Sweden, and Hamburg Observatory, Germany),  Christopher Thom (University of Chicago, USA, and Swinburne  University of Technlogy, Australia), Timothy C. Beers (Michigan State  University, USA), Jaehyon Rhee (Center for Space Astrophysics, Yonsei  University, Korea, and Caltech, USA).


[1]: The Hamburg/ESO sky survey is a collaborative project of the  Hamburger Sternwarte and ESO to provide spectral information for half  of the southern sky using photographic plates taken with the now  retired ESO-Schmidt telescope. These plates were digitized at  Hamburger Sternwarte.


Anna Frebel
McDonald Observatory, Texas
Phone: +1 512-461-7907

Norbert Christlieb
Department of Astronomy and Space Physics, Uppsala University, Sweden
Phone: +46-18-471-5982
Mobile: +49-176-67 67 14 08

Henri Boffin
ESO, Germany
Phone: +49 89 3200 6222

High-resolution images are available at